1. Field of the Invention
The present invention concerns a method of melt-spinning polyurethanes into textile decitex spandex and, more particularly, a method using ethylene glycol chain extender in the preparation of the polyurethane.
2. Discussion of Background Art
Spandex based on polymeric glycols, diisocyanates, and aliphatic diols is known. Ethylene glycol has been disclosed as a chain extender for making thermoplastic polyurethane-based spandex, but its use in making melt-spun spandex leads to the difficulty in melt-spinning such polyurethanes. U.S. Pat. No. 3,649,600 discloses spandex based on poly(tetramethyleneether) glycol 1,1xe2x80x2-methylenebis(4-isocyanatobenzene) and 1,4-butane-diol, polymerized in a two-step melt-polymerization process in the presence of 0.06-0.3% by weight of water based on the polymeric glycol. Although fibers are prepared, the high softening point of the polyurethanes obtained is expected to give poor spinnability due to excessive cross-linking at the high temperature needed to melt-spin the polymers.
Japanese Published Patent Application 04-146915 and U.S. Pat. No. 5,391,682 disclose a two-step melt polymerization and melt spinning of high molecular weight, number average above 160,000, polyurethanes under special conditions of high shear and a total glycol to isocyanate ratio range of 0.95 to 1.05. The product polyurethane is said to contain up to 500 crystalline particles (clumps) per kilogram.
Japanese Published Patent Application 05-214062 discloses melt polymerization and melt spinning of 2G-extended polyurethanes under special conditions of high shear rates (at least 10,000 secxe2x88x921) and very short residence times (60-300 seconds), without providing any examples using 2G, which can be used for pelletizing the large diameter strands. There is no disclosure of small-diameter textile fibers, whose manufacture makes more stringent demands on polymer quality.
There remains a need to be able to prepare polyurethanes based on ethylene glycol chain extender which can be reliably melt-spun into textile decitex fibers having useful properties.
The process of this invention for preparing melt-spun spandex comprises the steps of:
a) contacting in a solvent
i) a polymeric glycol selected from the group consisting of poly(tetramethyleneether) glycol and poly(tetramethyleneether-co-3-methyltetramethylene-ether) glycol having a number average molecular weight of 2400-8000,
ii) 1,11xe2x80x2-methylenebis(4-isocyanatobenzene) at a molar addition ratio of polymeric glycol to isocyanate of 1.8-6.0:1 and
iii) a chain extender selected from the group consisting of ethylene glycol, 1,3-propanediol, and 1,4-butanediol, to form a polyurethane;
b) adding a monofunctional chain terminator in order to obtain a polyurethane having a number average molecular weight 40,000-150,000 and a softening point of not more than 230xc2x0 C.;
c) removing the solvent; and
d) melt-spinning the polyurethane to form spandex.
As used herein, the term xe2x80x9cspandexxe2x80x9d has its customary meaning, that is, a manufactured fiber in which the fiber-forming substance is a long chain synthetic elastomer comprised of at least 85% by weight of a segmented polyurethane. The segments in the polyurethane are classified into xe2x80x9csoftxe2x80x9d segments of polyether-urethane blocks and xe2x80x9chardxe2x80x9d segments containing aromatic and urethane groups.
Suitable polymeric glycols include poly(1,4-tetramethyleneether glycol) (PO4G) and copolymers of tetrahydrofuran and 3-methyltetrahydrofuran (3MePO4G) having a number-average molecular weight of about 2400-8000. 3MePO4G having 4-20 mole percent 3-methyl moieties and a molecular weight of about 2400-4500 is preferred. When the molecular weight of polymeric glycol is less than 2400, the recovery properties of the resulting yarn tend to become low and it becomes difficult to obtain the desired elongation. When the molecular weight of the polymeric glycol exceeds 8000, the strength decreases and the resistance to chemicals declines. Use of the preferred 3MePO4G glycol gives spandex having a particularly good balance of mechanical properties.
The diisocyanate used in the process of this invention is 1,1xe2x80x2-methylenebis(4-isocyanatobenzene) (MDI). Other diisocyanates can be included in minor amounts, provided they do not compromise the advantageous effects of the invention.
The chain extender used in the method of the present invention is selected from the group consisting of ethylene glycol (abbreviated hereinafter as 2G), 1,3-propanediol (3G) and 1,4-butanediol (4G). 2G is preferred and presents the most demands on stable melt-spinning of polyurethanes prepared with it. This is because polyurethanes made with 2G have a high hard segment melting point (about 240xc2x0 C.). The hard ratio (the molar ratio of 2G to polymeric glycol) is in the range of about 1.75 to 3.0. Other diol chain extenders can be included in minor amounts.
A monofunctional chain terminator is used in the process of the present invention to control the molecular weight of the polyurethane. For polyurethanes useful in melt-spinning spandex, this number average molecular weight is determined to be 40,000-150,000. Useful chain terminators include monoamines, monoalcohols and monoisocyanates. Aliphatic alcohols are preferred, and n-butanol is especially preferred. The chain terminator can be added at the beginning or the end of the polymerization. Addition at the end of the polymerization is preferred for more uniform molecular weight distribution.
Any suitable solvent can be used in the polymerization process of the present invention, such as dimethylacetamide (xe2x80x9cDMAcxe2x80x9d), dimethylformamide, dimethylsulfoxide, n-methylpyrrolidone, and mixtures thereof.
The ingredients for the solution polymerization can be added in one step or in two steps. In the one-step method, the polymeric glycol, MDI and diol chain extender are added substantially simultaneously. In the two-step method, the polymeric glycol and MDI are first mixed to allow the formation of an isocyanate-terminated prepolymer, after which the diol chain extender is added to form a polyurethane. The one-step method is preferred because of its simplicity and low cost. In the one-step method, good results can be obtained when the starting materials are added to the solution at a relatively low temperature (for example in the range of about 20xc2x0 C.-30xc2x0 C.), the temperature is raised to the reaction temperature (for example at least about 70xc2x0 C.) and, once a preselected degree of polymerization has been reached, the chain terminator is added.
After the completion of the above steps, any suitable method can be used to remove the solvent from the polymer solution. Typically the solvent can be removed in a vacuum or with hot air, and, alternatively, the solution can be poured into steam or water. A devolatilizing extruder, such as that used in Canadian Patent 1,321,852, can also be used so that polyurethanes can be continuously polymerized and spun. Methods combining these approaches can also be used.
Polyurethane from which the solvent has been removed in this way can be generally processed into chips, flakes, particles and the like. Chips offer the advantage of being easy to transport, in addition to which any further required drying is easy. Because the resulting polyurethane is sometimes tacky, the addition of a suitable lubricant or antitack agent such as silicone oil, calcium stearate, sodium stearate, magnesium stearate, talc, barium sulfate, and the like can be useful. These lubricants and antitack agents can be added to the solution or during the course of solvent removal.
The polyurethane which has been desolvated in this way is optionally further dried as needed. Any conventional drying method can be employed without particular limitation as to either the drying conditions or the method. Particularly advantageous methods include vacuum drying and drying in heated dry nitrogen.
The solution polymerization process of the invention requires a molar ratio of diisocyanate to polymeric glycol (the xe2x80x9caddition ratioxe2x80x9d) of about 1.8-6. The resulting polyurethane has a softening point of not more than about 230xc2x0 C. (preferably not more than 220xc2x0 C.) and a number average molecular weight of about 40,000-150,000. When the addition ratio is less than about 1.8, the melting point of the spandex is unsatisfactorily low, and when it exceeds about 6, the elongation of the spandex decreases too much and the hand of the spandex becomes hard. An addition ratio in the range of about 2.5-3.7 is preferred for improved properties such as high softening point, high recovery, and good heat settability. When the softening point exceeds 230xc2x0 C., melt spinning becomes difficult to carry out and heat-settability decreases. When the number-average molecular weight is less than about 40,000, the strength and elongation tend to below and the yarn becomes fragile. When the molecular weight is greater than about 150,000, the elongation becomes low, and filter clogging can occur during melt-spinning.
The polyurethane that has been thus prepared is then melt-spun to form spandex. Any conventional melt spinning method can be employed.
The cross-section of the spandex of the present invention can be circular, flattened, or of any other suitable cross-section. The fiber decitex can be in the range of about 5-2500 dtex and preferably in the range of about 10-135 dtex. The spandex of this invention can comprise a single filament or two or more united filaments.
Various stabilizers, pigments and the like can be added to the polyurethane, even in the spinning step. Useful additives include light stabilizers and antioxidants such as hindered phenols, including 2,6-di-t-butyl-4-methylphenol and Sumilizer GA-80 (manufactured by Sumitomo Chemical Co., Ltd., Tokyo, Japan), benzotriazoles (e.g., various Tinuvins, made by Ciba Specialties), phosphorus-based chemicals (e.g., Sumilizer P-16, manufactured by Sumitomo), and amine-type chemicals (various Tinuvins); inorganic pigments such as titanium oxide, zinc oxide and carbon black; clay minerals such as montmorillonite; metallic soaps such as magnesium stearate; microbicides and deodorants such as silver, zinc and compounds thereof; lubricants such as silicones and mineral oils; and various antistatic agents such as barium sulfate, cerium oxide, betaine and phosphoric acid compounds. These additives can be added during the spinning step or in the polymerization step.
It is preferred that the fiber thus obtained be constant-length or relaxation heat-treated in an additional step in order to increase its elongation-at-break. It is more preferred that heat treatment be performed on relaxed fiber, during which the relaxation ratio (the ratio of feed roll rpm to takeup roll rpm during heat treatment) is not more than about 40%. Heat treatment can be carried out by treatment with dry heat or wet heat, for example by steam or an infrared heater. The heat-treatment temperature can vary according to the treatment medium, but is generally in the range from about 70xc2x0 C. up to about 5xc2x0 C. below the polyurethane softening point. When dry heat is used, the temperature is in the range from about 90xc2x0 C. to about 10xc2x0 C. below the polyurethane softening point. Such heat treatment can be carried out in the spinning step or in a later step and can be carried out in one step or in multiple steps.
The number-average molecular weight of the polyurethanes made by the method this invention was measured by gel permeation chromatography (GPC), using a polystyrene standard. xe2x80x9cSoftening point,xe2x80x9d as used herein, refers to the value measured by thermal mechanical analysis (TMA) of a dry film obtained by casting the polyurethane solution of the invention into a film which is then dried at 1200C. Polymer solution viscosity was determined in accordance with the general method of ASTM D1343-69 with a Model DV-8 Falling Ball Viscometer, (made by Duratech Corp., Waynesboro, Va.), operated at 40xc2x0 C.
Mechanical properties of the spandex were measured by using an Instron 4502 tensile tester. Five-cm long fiber samples were extended to 300% elongation at a rate of 50 cm/min and allowed to relax. This cycle was repeated five times, and then the sample were held at 300% elongation for 30 seconds. xe2x80x9cStress relaxation ratioxe2x80x9d was the percent reduction in stress after holding the sample at the final extended length for 30 seconds following the fifth 300% extension. The sample was subsequently allowed to fully relax from the extension.
The ratio of the sample length when the stress became zero following relaxation from extension to the length before extension was the xe2x80x9cset ratioxe2x80x9d. Tenacity at break and elongation at break were, respectively, the elongation and stress when, in the above measurement procedure, the fiber was extended a sixth time until it broke.
The hysteresis ratio was the stress, measured at 200% extension on the stretch part of the fifth 0-300% stretch-and-relax cycle, divided by the stress at 200% extension on the relax part of the fifth cycle after the extended length had been held for 30 seconds and then released.
To determine the heat-setting percent, the fiber was treated in a free, unsecured state for 10 minutes with 100xc2x0 C. steam, then treated in an unsecured state for 2 hours with boiling water, and then dried at room temperature for one day. The fiber was then extended 100%, treated with 115xc2x0 C. steam for 1 minute, and subsequently treated with 130xc2x0 C. air for 1 minute at the same degree of extension. Next, the fiber was left to stand at room temperature for one day, and the final length was measured. The heat setting percent was calculated as the final length divided by the length at 100% extension, multiplied by 100.